Apple: iOS

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The iOS kernel is used in the iPhone, iPad and iPod. It uses the XNU1) The OS X kernel. The acronym stands for X is Not Unix. XNU combines the functionality of Mach and BSD with the I/O Kit, the driver model for OS X Apple Developer Glossary: XNU kernel of Darwin2) Another name for the core of the OS X operating system. The Darwin kernel is equivalent to the OS X kernel plus the BSD libraries and commands essential to the BSD command-line environment. Darwin is open source technology. Apple Developer Glossary: Darwin .

Darwin is an open sourceUnix-like computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects.3) “Apple releases OS X 10.10 Yosemite Open Source Darwin code”, Seth Weintraub, 31 October 2014, https://9to5mac.com/2014/10/31/apple-releases-os-10-10-yosemite-open-source-darwin-code/

Darwin forms the core set of components upon which OS X and iOS are based. It is mostly POSIX compatible, but has never, by itself, been certified as being compatible with any version of POSIX. (OS X, since Leopard, has been certified as compatible with the Single UNIX Specification version 3 (SUSv3).